fix: improve thread-safe and atomic writing

[vazarkevych]

Summary

This PR addresses thread-safety issues across several core components of the GrowthBook Java SDK.

Changes

GBFeaturesRepository

• Replaced mutable fields with AtomicBoolean, AtomicLong, AtomicReference where appropriate
• Changed ArrayList for refresh callbacks to CopyOnWriteArrayList
• Marked refreshStrategy and sseRequest as volatile
• Replaced sseHttpClient and sseEventSource plain fields with AtomicReference

FileCachingManagerImpl

• Replaced direct file writes with write-to-temp + atomic Files.move(ATOMIC_MOVE) pattern to prevent partial reads during concurrent access
• Wrapped both saveContent and loadCache with class-level synchronization

GrowthBook

• Replaced HashMap with ConcurrentHashMap for assigned experiments map
• Replaced ArrayList with CopyOnWriteArrayList for callbacks
• Marked callbacks and evaluationContext as volatile

GrowthBookClient

• Marked static repository field as volatile
• Wrapped initialize() with synchronized block to prevent double-initialization
• Replaced HashMap/ArrayList with ConcurrentHashMap/CopyOnWriteArrayList
• Marked globalContext as volatile

GrowthBookUtils

• Extracted fireSubscriptions from both GrowthBook and GrowthBookClient into a shared utility method to eliminate duplication

InMemoryStickyBucketServiceImpl

• Minor synchronization improvements

NativeJavaGbFeatureRepository

• Fixed GbCacheManager initialization order